1. Technical Field
The present invention relates to heat transfer on a large scale, and more particularly to materials that improve heat transfer rates.
2. Description of Related Art
Nuclear reactors generally include a vessel in which a nuclear reaction takes place. The vessel generally includes a vessel envelope that encloses a core of nuclear material, control rods, working fluid and the like. Often, the vessel is housed in a large building, described herein as a containment envelope. The containment envelope is generally much larger than the vessel, and is generally sealed or sealable to the outside world up to a maximum pressure, and filled with a gas such as air.
Notwithstanding safety measures, uncontrolled nuclear reactions may result in the generation of heat within the vessel beyond the ability of the heat transfer apparatus to remove the heat. In such cases, the core may fail, and temperatures within the vessel may rise to temperatures above 500, 1000, 1500, 2000, 2500, or even 3000 degrees Celsius.
In some cases, the vessel and associated apparatus may transfer sufficient heat from a failed core that the vessel envelope remains substantially intact. In other cases, the failed core material may breach the vessel and enter the containment environment. Vessel envelopes may be designed to resist temperatures up to 600, 800, 1000, 1200 or even 1400 degrees Celsius, but core materials may reach temperatures over 2400 degrees Celsius.
The mass of various core materials and vessel materials may be very large (e.g., hundreds or even thousands of tons). As such, the trajectory of the ex-vessel core material, (often described as “corium”) from the vessel to a surface of the containment is largely influenced by gravity (e.g., the corium may fall to a “floor” of the containment envelope). Contact between the corium and containment envelope typically degrades the containment envelope and associated components, often due to the high temperature of the corium.
The removal of heat from the corium should occur at a faster rate than the generation of heat by the corium. The removal of heat from the corium should also be faster than a degradation of the containment envelope in order to prevent or delay a breach of the containment envelope.
Cooling liquids (e.g., water) may contact the corium and remove heat from the corium via evaporation of the liquid. However, evaporation generally increases the total gas pressure inside the containment, and so evaporated liquids (e.g., steam) may generate significant hydrostatic pressure in the containment (e.g., greater than 3, 7, 10, 15, 20, 30, 40, 50, or even 100 atmospheres) which may exceed the maximum pressure that the containment envelope can contain. Increasing the size of the containment envelope may increase the rate of condensation (in that the total mass of condensing fluid generally increases with increasing surface area upon which it is condensing), but increasing the size of the containment envelope too much may cause other problems, such as reduced resistance to internal pressure. For a given containment envelope inner surface area, increasing a condensation rate of a vapor phase (on the inner surface) that has (in evaporating) removed heat from the core material may be desirable.
To reduce pressure within the containment envelope, heat absorbed by evaporation may be transferred (e.g., to the outside world) before the pressure inside the containment envelope rises above a critical pressure associated with failure of the containment envelope.
Condensation of a gas or vapor may transfer heat from the vapor to a surface. However, many materials that are liquids at room temperature may have condensation rates on an inner surface of the containment envelope that are slower than their associated evaporation rates at the corium. For example, water may condense at approximately 100 degrees, and an inner surface temperature may be 30 degrees, but ex-vessel material (evaporating the water) may be 2900 degrees. A condensation rate may be mismatched with an evaporation rate when the boiling point (or the condensation temperature) is relatively close to the temperature of the inner surface and much lower (e.g., 500, 1000, 1500, 2000, or even 2500 degrees lower) than the first temperature and/or temperature of the ex-vessel core material.
When a gaseous boundary layer is formed between hot, ex-vessel core material at a first temperature and a liquid having a boiling point far below the first temperature (e.g., water cooling of 2400 degree core material), heat transfer to the liquid may be relatively reduced by a gaseous boundary layer. In such cases, heat transfer from the core material may not necessarily “benefit” from water's boiling point being hundreds, or even thousands of degrees below the core material temperature. As such, using a cooling liquid with a higher boiling point may result in rates of heat transfer from the core material to the liquid that are at least as fast as heat transfer to water.
Thus, the removal of heat from an ex-vessel mass of core material, in a manner that does not lead to containment overpressure and/or failure may reduce the risk of containment breach.